This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to transmission and phase synchronization of a detected free induction decay (FID) signal to signal processing apparatus in a MRI system.
Magnetic resonance imaging (MRI) requires placing an object to be imaged in a static magnetic field, exciting nuclear spins in the object within the magnetic field, and then detecting signals emitted by the excited spins as they precess within the magnetic field. Through the use of magnetic gradient and phase encoding of the excited magnetization, detected signals can be spatially localized in three dimensions.
FIG. 1A is a perspective view partially in section illustrating conventional coil apparatus in an NMR imaging system, and FIGS. 1B–1D illustrate field gradients which can be produced in the apparatus of FIG. 1A. This apparatus is discussed by Hinshaw and Lent in “An Introduction to NMR Imaging: From the Block Equation to the Imaging Equation,” Proceedings of the IEEE, Vol. 71, No. 3, March 1983, pp. 338–350. Briefly, the uniform static field B0 is generated by the magnet comprising the coil pair 10. A gradient field G(x) is generated by a complex gradient coil set which can be wound on the cylinder 12. An RF field B1 is generated by a saddle coil 14. A patient undergoing imaging would be positioned within the saddle coil 14.
In FIG. 1B an X gradient field is shown which is parallel to the static field B.sub.0 and varies linearly with distance along the X axis but ideally does not vary with distance along the Y or Z axes. FIGS. 1C and 1D are similar representations of the Y gradient and Z gradient fields, respectively.
FIG. 2 is a functional block diagram of conventional imaging apparatus. A computer 20 is programmed to control the operation of the NMR apparatus and process FID signals detected therefrom. The gradient field is energized by a gradient amplifier 22, and the RF coils 26 for impressing an RF magnetic moment at the Larmor frequency are controlled by the transmitter 24. After the selected nuclei have been flipped, the RF coil 26 is employed to detect the FID signal which is passed to the receiver 28 and thence through digitizer 30 for processing by computer 20.
Heretofore, the detected FID signals have been coupled to the signal processor via coaxial cables. However, cable transmission can degrade the detected signal, and coaxial cables require the use of balans for signal coupling. Further, cables present a contamination problem in interventional MRI which requires a sterilized coil.
Wireless transmission of the detected FID signals to the processor has been proposed to eliminate the need for cables. See for example, Leussler U.S. Pat. No. 5,245,288 and Murakami et al. U.S. Pat. No. 5,384,536. Since frequency and phase of the detected signals are critical in signal processing, Leussler proposes transmitting an auxiliary signal from the signal processor to the signal detector for use in a local oscillator for frequency shifting the FID signal for transmission. The processor can then compensate for any phase errors introduced through signal modulation by comparing the received modulation signal to the transmitted auxiliary signal. Since the detector oscillator frequency is generated from this auxiliary signal, it is assumed that no phase errors are incurred.
Murakami et al. propose a similar wireless MRI system which employs a reference signal transmitted from the processor to the detector for use in frequency conversion (modulation) of the detected FID signal for transmission to the processor.
Unfortunately, the detected auxiliary signal suffers fluctuations when received by the detector electronics due, in part, to the relatively high voltage pulses of the MRI pulse sequence RF transmitter modulated FID signal. The phase locked loop driven by the auxiliary signal in controlling the local oscillator suffers instability in operation and requires continual phase locking. This electronics has a finite recovery time and will suffer phase skips due to counting errors in the phase locked loop. Moreover, the extra locking circuitry on the detector adds to the detector complexity.
Basically, whenever a FID signal undergoes a frequency change due to mixing with a local oscillator, a phase shift is created. The present invention is directed to providing wireless transmission of FID signals to a processor which can be readily compensated for phase variations in the FID detector and in transmission.